Hydraulic tensioners are particularly used for maintaining the pre-stress for a traction drive, i.e., a belt or chain drive, as constant as possible when elongation has taken place, and to effectively damp vibrations.
A hydraulic tensioner of the generic type known from U.S. Pat. No. 4,790,801 comprises a piston slidably guided in a cylinder which is positionally fixed in an outer housing, said housing being pivoted by a fixing eye, for example, on a crankcase of an internal combustion engine. At its end turned away from the cylinder, the piston is fixed in a piston housing which engages partly into the outer housing and is connected by a fixing eye directly or indirectly to a tension roller which bears against the traction drive. A compression spring coaxially surrounding the piston and the cylinder is arranged between the outer housing and the piston housing to spring load the piston housing so as to urge it towards the tension roller. The construction of the hydraulic tensioner further permits an exchange of hydraulic fluid volume from a high pressure chamber in the cylinder as a result of piston motion. Depending on the direction of motion of the piston, hydraulic fluid can flow from the reservoir into the cylinder through a one-way valve arranged on the cylinder, or on a reversal of the piston motion, the hydraulic fluid can be displaced from the high pressure chamber into the reservoir through a leak gap situated between the piston and the inner wall of the cylinder. To seal the telescoped components, i.e. the outer housing and the piston housing which is partly slidable therein, a bellows seal is provided which bears under pre-stress against the piston housing and the outer housing, each end of the seal being positionally fixed by a retention bead which engages into a circumferential groove. The prior art bellows seal offers no possibility of reducing overpressure in the outer housing which can occur, for example, on a variation of length of the traction drive or during mounting, i.e. during belt replacement, when the piston of the tensioner is pressed to a block. The pushing-in of the piston creates an overpressure with a concomitant inflation of the bellows seal which often results in a displacement of the bellows seal, i.e. in a radial exit of the retention bead out of the circumferential groove and a subsequent axial displacement, so that an exact guidance of the bellows seal is no longer possible and the danger of destruction exists.